The present invention relates to auto-focusing and, more particularly, to auto-focusing by use of a solid-state imaging device.
Auto-focusing is initiated by projecting an image through a lens onto a detection device which is made up of imaging elements. The lens is, then, focussed according to a comparison of image signals generated by the imaging elements. Since auto-focusing is performed on the basis of the detected image, it is important to accurately detect the image projected onto the detection device.
The detection device for detecting the image may be embodied, for example, as a Charge Coupled Device (CCD) linear sensor 100 shown in FIG. 11 which is a solid-state imaging device. Fundamentally, the CCD linear sensor includes light receiving units 101, i.e., the imaging elements, which receive the projected image and generate a pixel output signal V.sub.out composed of a series of voltage signals representing pixels of the image. The exposure time, that is, the time in which the CCD linear sensor is exposed to light, is determined by the level of a peak value PH.sub.out for the pixel voltages. Thus, the pixel output signal V.sub.out is employed to generate an auto-focusing signal and the signal PH.sub.out is employed to control the exposure time so that an output level of the pixel output signal V.sub.out is appropriate for auto-focusing.
The light receiving units 101 of the CCD linear sensor that receive the image comprise a sensor array 102. It will be appreciated that the sensor array may be linear, i.e., one-dimensional, because auto-focusing does not necessarily require detecting signals for an entire, i.e., two-dimensional, image. That is, the CCD linear sensor need only receive a single line of the image and it is sufficient that the sensor array is linear. Each light receiving unit in the sensor array converts incident light into an electric charge and accumulates the electric charge during the exposure time. The amount of charge in each light receiving unit corresponds to the amount of incident light accumulated over the exposure time and these accumulated charges, therefore, represent the image along the linear sensor array.
A read-out gate controlled by a read-out gate pulse .phi.ROG initiates the reading of the charges. The charges are read out by transferring each charge from the sensor array of light receiving units; and the transfer of charges is timed by transfer pulses .phi.H.sub.1, .phi.H.sub.2, which are essentially clocks that trigger the light receiving units, the read-out gate and the charge transfer register to transfer the accumulated charges. The shifted charges are transmitted serially, via charge transfer register 104, to a charge.backslash.voltage converting unit 105. The charge.backslash.voltage converting unit converts each accumulated charge into a corresponding voltage and stores these voltages in a buffer 106. The buffer holds these voltages for transmission to the output terminal 107 as the output signal CCD.sub.out from the CCD linear sensor.
A buffer 111 stores and holds the output signal CCD.sub.out at an output thereof as the pixel signals V.sub.out. The pixel signals V.sub.out are a series of voltage signals representing the accumulated charge in each of the light receiving units, or pixels. Auto-focusing is achieved, for example, by comparing the signal levels of each pixel voltage in the pixel output signal V.sub.out. This comparison cannot be made, however, when the light receiving units receive too much light. When the light incident on the light receiving units becomes too great, for example, the accumulated charges reach their maximum and the light receiving elements become saturated. As a result, it is not possible to derive a contrast therefrom and auto-focusing is not possible.
It shall be noticed that the amount of accumulated charge for each light receiving unit varies with the exposure time. To resolve the saturation problem, therefore, the exposure time may be controlled to limit the amount of accumulated charge in each light receiving unit. The exposure time is controlled by, for example, controlling a shutter speed of the CCD linear sensor. One way to obtain the exposure time is to monitor the peak value for a given series of pixel voltages and modify the exposure time for the next cycle if the peak value is outside an acceptable range.
The circuit of FIG. 11 provides a peak detecting unit 114 which detects and holds the peak value using a diode 112 and a capacitor 113. A buffer 115 stores the peak value for transmission to a buffered output PH.sub.out. As shown in FIG. 12, the peak value PH.sub.out follows the peak values for the pixel voltages V.sub.out, thus yielding the overall peak value for all of the pixel voltages.
The peak detecting unit 114 of FIG. 11 may be constructed as the MOS transistor circuit shown in FIG. 13. The pixel signals CCD.sub.out of the CCD linear sensor (100, FIG. 11) are received as an input signal V.sub.in. The input pixel signals V.sub.in are sent to a source follower circuit 121 comprising an input stage of N-channel MOS transistors Q51 and Q52. The input stage functions as a buffer, generates the signal V.sub.out and forwards the same to a peak detecting unit 122. The peak detecting unit is comprised of a diode Q53 and a capacitor C; the diode Q53 is formed by a P-channel MOS transistor with its gate connected to its source. The peak detecting unit 122 detects the peak value PH.sub.out of the pixel signals V.sub.out and forwards this peak value to an output stage 123. The output stage is another source follower circuit and is composed of N-channel MOS transistors Q54 and Q55 which buffer the peak value PH.sub.out.
The devices of FIGS. 11 and 13 are disadvantageous because they cannot determine the correct exposure time accurately. When the difference in light intensity between a target object to be focused and surrounding objects is great, for example, the contrast is too high to detect and, as a result, the correct exposure time cannot be determined. To explain, when the target object has a low signal level and the surrounding objects have a high signal level, the peak detecting unit will detect the high signal levels of the surrounding objects and the exposure time will be greatly reduced to decrease the exposure to the high signal levels. Since the signal level of the target object is already low, greatly reducing the exposure time will decrease the target object signal level below detectable levels. As a result, a detectable contrast between the target object and the surrounding objects cannot be ascertained and the correct exposure time cannot be determined. Consequently, accurate auto-focusing cannot be achieved with the devices of FIGS. 11 and 13.
Another disadvantage of the devices of FIGS. 11 and 13 is that auto-focusing cannot be performed accurately for a target object. The devices of FIGS. 11 and 13 do not detect the peak value for only the target object, but detect the peak value for all of the objects in the image projected on the sensor array 102. Therefore, it is not possible to perform accurate auto-focusing for only the target object in the image.